1. Field of the Invention
The present invention relates to a method for printing an image by using an ink jet printing apparatus for ejecting ink from a printing head to a printing medium, particularly to a method for printing an image by a serial type ink jet printing apparatus while adopting a multi-scan system.
2. Description of the Related Art
There are various image-forming apparatuses for forming image information (including characters, symbols or others), such as those having a printing function, a copying function or a facsimile function, those integral with a computer or a word processor, or those used as output devices for a work station. In such image-printing apparatuses, the printing operation is carried out on a printing medium such as paper or a plastic sheet (a transparency film), based on the image information by using printing means of a predetermined system. As representatives of such a printing system applied to this printing means, for example, an ink jet system, a wire dot system, a heat-sensitive system, a thermal transfer system and an electro-photographic system are listed.
Of them, the ink jet system operates to eject ink from the printing head, which is printing means, to the printing medium to form the image thereon. According to this system, it is possible to print a high-precision image at a high speed while making the printing head more compact than those in the other systems. Also, by carrying a plurality of color ink heads simultaneously thereon, it is possible to relatively easily realize colorization. Further, since the printing is done directly on a plain paper, the running cost is low, and since this system is a non-impact system, noise is less. In view of such merits, the printing apparatus of the ink jet system has recently been widely used in personal users.
The ink jet system is classified into several types in accordance with means for generating energy used for ejecting ink. Of them, a thermal ink jet system is most popular. In the thermal ink jet system, an electro-thermal converter; i.e., a so-called heater; is disposed in an ink passage of the respective printing element (hereinafter also referred to as a nozzle). A bubble is generated in the ink passage by the application of voltage to the heater, and used as the energy for ejecting ink. According to the printing head of this system, during the production process, steps for producing a semiconductor are carried out, such as an etching, a deposition or a sputtering, to provide electro-thermal converters or electrodes by films on a substrate and form liquid passage walls and top walls thereon. Thereby, it is possible to arrange ink passages at a relatively high density. Also, by using the advantages of an IC technology or a micro-processing technology, it is possible to realize the prolongation or the planarization of the printing head. Accordingly, the thermal ink jet system is advantageous in that it is responsible even to the recent requirement for a high-speed and high-resolutional printed image by the adaptation of the structure of the printing head.
The ink jet printing apparatus is classified into a line type and a serial type in accordance with the printing style thereof. The serial type mainly prevails among personal users because of its small size and low cost. The serial type printing apparatus sequentially forms the image on the printing medium by alternately repeating a main printing scan for moving a carriage mounting the printing head thereon relative to the printing medium while ejecting ink from the printing head and a sub-scan for conveying the printing medium by a predetermined amount in the direction orthogonal to the main printing scan, in this case, a width of an area in which the image is to be printed by one main printing scan is determined in accordance with the arrangement density and a number of a plurality of ink ejection orifices provided in the printing head. Accordingly, if the printing operation is proceeded by repeating the main printing scan for covering that width and the sub-scan in correspondence to that width, the image will be completed in the shortest time. Practically, however, a so-called multi-scan system is often adopted for the purpose of further improving the image quality.
The explanation will be briefly made on the multi-scan system below. In the multi-scan system, the main printing scan is executed N times (N≧2) in the area which would be printed by one main printing scan. An amount of the sub-scan carried out between the subsequent main printing scans corresponds to a printing width of a plurality of printing elements contained in each block when the number of printing elements arranged in the printing head is divided into N blocks. That is, the image in the same image area is formed by the printing elements contained in N blocks through N printing scans.
In general, the number of printing elements contained in the respective block is the same when divided into N blocks. This, however, is not limitative. For example, when a total number of the printing elements cannot be divided by N, each of the blocks Nos.1 to N−1 may be composed of M elements (M is an optional number) and the final block No. N may be composed of the residual number of elements. Alternatively, by sequentially repeating M elements and N elements (M and N are optional numbers, respectively), the printing width in the going direction (the direction of the odd number scan) and that in the returning direction (the direction of the even number) may be equalized to each other. Further, for example, the printing head having ten printing elements may be divided into three blocks of two, six and two printing elements, wherein the areas, each printed by the two printing elements located at the respective ends, are solely subjected to twice the printing scans of the multi-scan system. In this case, the area printed by the six printing elements located in a central region is completed by a single printing scan, whereby the multi-scan number may be represented by N =1.5 times.
Since the image is completed by a plurality of printing scans executed by different blocks according to the multi-scan system, all of the printable image data are not printed by one main printing scan. Thus, a mask is used for distributing image data to the respective blocks. The mask is often determined independently from the image signal. For example, it is possible to form the construction for determining whether or not the image signal given by the respective printing scan is to be printed.
At this time, as seen from the individual image data, a probability to be printed by one main printing scan is determined by this mask. In other words, the image data to be printed are thinned to a certain extent by the mask, wherein the thinning probability is referred to as a thinning percentage in this text. The thinning percentage is meant in reverse to a probability for printing the data during the respective printing scan (hereinafter referred to as a printing percentage).
One concrete example of the multi-scan system in accordance with the above structure will be cited below. When the multi-scan printing is carried out four times while using a hundred printing elements, the printing elements are divided into four blocks, each consisting of 25 printing elements. An amount of the sub-scan between the subsequent printing scans corresponds to 25 printing elements. The mask corresponding to the respective block in the respective printing scan has the thinning percentage of 75% and the printing percentage of 25%. The mast patterns are complemental each other between the four blocks, and by overlaying the four mask patterns with each other, the 100% printing is carried out In this regard, although the description has been made, as a general example, so that one hundred printing elements are evenly divided by the multi-scan number N=4, the multi-scan system should not, of course, be limited thereto. As described before, the multi-scan number N may not completely divide the total number of the printing elements. If the main printing scan is carried out by a plurality of different blocks, the multi-scan system is realized.
The technology disclosing the basic structure and effect of the above-mentioned multi-scan system will be introduced below.
Japanese Patent Application Laid-open No. 55-113573 (1980) discloses a structure for complete the image by two printing scans of going and returning passages. The mask pattern applied to the respective printing scan is limited to an alternate lattice-like checker pattern both in the vertical and horizontal directions, and the adjacent dots are not printed by the same printing scan. A dot printed by the second printing scan is applied before a dot which has been printed by the first printing scan completely dries to prevent the dot from deforming. In this case, the thinning percentage of the respective main printing scan is limited to 50% by the checker pattern, and there is no description about the conveying amount in the sub-scanning direction. Accordingly, the effect for smoothing the overall image as in the above-mentioned multi-scan system is not disclosed in this document.
Japanese Patent Application Laid-open No. 58-194541 (1983) discloses a method for preventing the color-banding during the bi-directional printing in the serial type color printer. In this method, a printing head having a plurality of rows of printing elements arranged parallel to each other is used for carrying out the reciprocation printing scan. At this time, dots less than total dots to be printed are intermittently printed in the going passage, while the residual dots are intermittently printed in the returning passage. Thereby, it is possible to arrange the dots, each different in the overlaying order from the other, to be uniformly distributed in an area by the overlaid printing of the plurality of rows of the printing elements. Accordingly, it is possible to prevent the deviation of the color tone in the printed image mainly derived from the overlaying of color inks. In this case, the main purpose is to avoid the color-banding, and there is no description about positions of dots to be printed by the respective printing scans. In the disclosed embodiment, mask patterns other than the checker pattern are described, such as a transverse thinning pattern for alternately printing solely in the vertical direction or a vertical thinning pattern for repeating the thinning printing solely in the transverse direction.
The U.S. Pat. No. 4,748,453 discloses a printing method carried out on a printing medium having a low ink-absorption rate such as an OHP sheet (transparency film) When the printing is carried out on the same area by first and second (or more) printing scans, pixels located alternately in the horizontal and vertical directions are solely printed in the same printing scan, and then the complemental printing is carried out in the subsequent printing scan so that is the beading of ink is avoided on the printing medium having a low ink-absorption rate. Also, if a color image is formed, in the same manner as in the above-mentioned Japanese Patent Application Laid-open No. 58-194541 (1983), the order of the ink ejection to mixed color pixels is reversed between the first and second printing scans (in other words, the reciprocation printing is carried out), whereby there is also an effect for avoiding the color-banding. Since a main object of the U.S. Pat. No. 4,748,453 resides in the avoidance of the beading between the respective pixels, the pixels printed by one scan are alternately arranged both in the horizontal and vertical directions (that is, pixels are not adjacent to each other).
A feature common to these three patent documents described above is that the same image area is completed by a plurality of printing scans, which could be said as the basic feature of the multi-scan system. However, in either case, it is adapted that the adjacent dots are not simultaneously formed or the dots to be printed are distributed as far and uniformly as possible between the respective printing scans. As a mask pattern, a checker pattern or a simple vertical or transverse thinning pattern is used common to the respective colors.
By adopting the multi-scan system described above, it is possible to obtain, not only the effects disclosed in the three patent documents, but also other effects in that the variation of the printed positions due to the printing elements or that of the amounts of the sub-scan is spread to smooth the image as a whole by conveying the printing medium at a predetermined amount between the main printing scans. Particularly, an effect for eliminating a so-called joint streak generated on the boundary between the adjacent printing scans is important, whereby the multi-scan system has been widely used at present in the serial type ink jet printing apparatus.
In this regard, if the printing percentage of the mask pattern and the amount of the sub-scan are reduced and conversely the number of multi-scans is increased, the conventional multi-scan system is further effective. That is, a smoother image is obtainable from four scans than two scans, or from eight scans than four scans. On the other hand, however, the increase in the number of multi-scans results in the increase in the number of printing scans and, therefore, the prolongation of the printing time. Accordingly, in the recent time, a structure is put into practice, in which a plurality of printing modes are provided in advance in the printing apparatus so that the user is selected a suitable one therefrom in accordance with kinds and/or uses of the printed image.
Further, according to the multi-scan system, it is possible to solve more problems and generate new effects by changing the mask pattern and the amount of the sub-scan while regulating the mutual relationship between the both. Conversely, there may be a case in which new problems arise by adopting the multi-scan system. Accordingly, many multi-scanning methods are recently proposed, using masks having various characteristics in accordance with the problems or objects to be solved.
Several prior arts which are the modification of the multi-scan system will be described below. In the serial type printing apparatus, there are a mono-directional printing in which the printing is carried out solely in the going printing scanning direction and a bi-directional printing in which the printing is alternately carried out both in the going and returning scanning directions. Of course, the bi-directional printing is more advantageous in view of the time cost than the other because the printing time is shortened by the backward scanning. In this case, however, a new problem called as color-banding generates in the color ink jet printing apparatus.
The color-banding is a problem generated due to the difference in order of ink colors to be printed in accordance with the directions of the printing scan. That is, even if the printing is carried out based on the same data, there is a difference in tint visible by naked eyes between images printed in the going passage and the returning passage.
Several countermeasures characterized by a mask have already been proposed for solving such color-banding. For example, according to Japanese Patent No. 3,200,143, a method for reducing the color difference between the going printing scan and the returning printing scan is disclosed, by carrying out the printing with different colors at different positions in the same printing scan, while using a mask characterized in that, in a plurality of thinning masks corresponding to different colors, the arrangement of pixels in at least one mask is different from that in the other thinning masks.
Also, according to Japanese Patent No. 3,236,034, there is the disclosure in that mask patterns are provided in fixed correspondence to a plurality of blocks, respectively, so that the mutually complemental relationship is maintained between the blocks, which relationship is the same both in first and second printing heads. According to this document, it is possible to mitigate the color-banding due to the deviation of the printing percentage in the respective printing scan caused by the relationship of the arrangement between the mask pattern and the image data, by fixing the mask pattern to the printing head.
Further, Japanese Patent Application Laid-open No. 2002-144552 discloses a structure of a mask pattern in the multi-scan system of three scans or more for approximately equalizing areas covered with initial two scans (a covered amount), based on a fact that a dominant color is mainly decided in the initial two scans. The above-cited Japanese Patent Application Laid-open No. 2002-144552 supposes that the color image is printed mainly with ink excellent in permeability, and is characterized in that the printing percentage in the first scan is made to be lower than that in the second scan to approximately equalize the cover amounts by the two scans.
In the above description, the prior arts for mainly solving the color-banding have been cited. However, for example, in Japanese Patent No. 3,093,489, the multi-scan method for positively solving the joint streak in addition to the color-banding is disclosed. In Japanese Patent No. 3,093,489, there is a description in that an image is completed in an image area by sequentially printing the thinned images having a predetermined printing ratio by the respective main scans, and in at least one of a plurality of main scans, the printing ratio to a pixel group in the boundary region between the adjacent image areas is made to be smaller than the predetermined printing ratio. This is because the joint streak is liable to occur in the boundary region since one more printing scan is repeated in this region than the other region, and therefore, the mask pattern is adapted to complete the image even in the boundary region by the same number of the printing scans as in the other regions, if possible.
Further, according to Japanese Patent Application Laid-open No. 2002-292910, a mask pattern for mitigating a drawback called as an end deflection is disclosed, which is peculiar to a case wherein ink droplets are ejected at a high speed and a high density. According to Japanese Patent Application Laid-open No. 2002-292910, since one cause of the end deflection is the high-density ejection of ink in an end region of the printing head, the printing percentage of the mask pattern to be applied to the end region of the printing head is to be lower than in the other region.
As described above, suitable mask patterns and multi-scanning methods are employed for solving various problems in the recent ink jet printing apparatus so that a high-quality image is obtainable.
In the conventional color ink jet printing apparatus, ink mainly composed of dyestuff and excellent in permeability has generally been used. In the color printing, it is important that different color inks are quickly absorbed in the printing medium without blotting each other on the printing medium. If the different color inks are brought into contact with each other prior to being absorbed in the printing medium, the mixing of the inks occurs to cause a defect called as a boundary blotting on the image.
Although the ink excellent in permeability has a drawback in that a printed dot becomes unnecessarily larger and a sufficient color density is difficult to be resulted, as well as this ink is inferior in clarity in comparison with ink low in permeability to be characteristic of subsequently overlaying the printed ink (hereinafter referred to as an overlay type ink in this text), Nonetheless the ink excellent in permeability has been often used in the prior art for avoiding the occurrence of the boundary blotting.
Recently, since a high-precision printing head has been developed, an amount of ink ejected from one printing element becomes very little. Accordingly, the boundary blotting on the printing medium has gradually become less problematic, and the situation for using the overlay type ink in the color printing has being established. In addition, since a new printing medium has been developed to realize the clearer printing free from the boundary blotting, the advantage of the color ink jet printing apparatus is capable of ejecting a small droplet of the overlay type ink is recently recognized again.
In the overlay type ink, not only dyestuff but also pigment is usable as a colorant. If the pigment is used, it is expected that various properties necessary for the printed image are enhanced, such as color density and clarity of the printed image, the image-reserving capacity such as water-resistance or light-resistance, whereby the value of the ink jet printing apparatus itself is up-graded.
However, the ink using pigment or the overlay type ink have problems peculiar thereto. One of them is a so-called bronzing. The bronzing is a phenomenon in that the printed image varies its tint or glossiness as a bronze product in accordance with the light-projecting direction or the image-viewing direction. To solve this bronzing, a method for producing the ink itself has already been improved. For example, see Japanese Patent Application Laid-open Nos. 7-247452 (1995), 6-228476 (1994), 7-268261 (1995) and 2002-069340, and Japanese Patent No. 3,249,878.
In practice, however, there is hardly a case in which the ink completely free from the bronzing as disclosed in the above prior art documents is solely used. This is because a limit exists in the application range of the ink due to various factors such as the ink-ejection characteristic of the printing head, the compatibility of the ink with a printing medium or the production cost of the ink.
Even though the ink is improved by the above-mentioned various methods for the production of ink, the effect thereof is insufficient for solving the problem relating to the ink glossiness in the recent color ink jet printing apparatus of a multi-scan system.
In the color ink jet printing apparatus using a plurality of color inks, the glossiness of the image is differently felt in accordance with ink color. It is also known that the glossiness varies by an amount of ink ejected to the printing medium (an application amount) and the degree of variation thereof is different in accordance with ink colors.
FIG. 1 shows the degree of glossiness of a plurality of color pigment inks, which is a physical value felt as the glossiness, measured while varying the ink-ejection amount. In FIG. 1, the abscissa axis represents the ink-ejection amount per unit area in the printing medium and the ordinate axis represents the degree of glossiness of the printing medium in the respective ink colors in correspondence to the respective ink-ejection amount. In this regard, the measurement was carried out by using a Gloss Checker IG-320 manufactured by K.K. Horiba Seisakusho. The calibration during the measurement is carried out by measuring a reference plate having the degree of glossiness of 90 determined in comparison with a surface of a glass plate standardized by JIS having a refractive index of 1.567 which degree of glossiness is defined as 100, while slanting the Gloss Checker at 60° together with a light source relative to a vertical line.
It is apparent that the difference exists in degree of glossiness between ink colors from results of the measurement shown in FIG. 1. It is also confirmed that, while the degree of glossiness becomes basically larger as the ink-ejection amount increases, the rising rate or changing rate thereof is different between the respective ink colors.
One of reasons for generating the difference in degree of glossiness in accordance with the ink colors is that the glossiness is relied on the aggregation of the colorant such as dyestuff or pigment and the degree thereof is different in accordance with molecular structures of the colorant. Further, the aggregation is accelerated by the contact of adjacent dots before the ejected ink is absorbed by the printing medium. Accordingly, the pigment type ink having a relatively low permeation speed is felt to have a higher degree of glossiness than the dyestuff type ink having a higher permeation speed. In addition, since the pigment type ink is liable to be influenced with the contact between adjacent ink dots as described above, the degree of glossiness is easily variable in accordance with the ink-ejection amount.
Reasons why the pigment type ink is low in permeability In comparison with the dyestuff type ink are as follows. One is that since the pigment type ink contains more resin component or oil component than in the dyestuff type ink, the viscosity or surface tension is relatively high. If the ink viscosity or surface tension is high, the permeation speed into the printing medium is also low. Even if the viscosity of the pigment type ink is made to be equalized to that of the dyestuff ink, the surface tension may increase. Further, the pigment particle has a relatively large diameter in comparison with the dyestuff particle. Even in the general aqueous pigment type ink, the particle diameter thereof reaches approximately 100 nm, whereby the permeation speed or fixing speed into the printing medium is lower than that of the dyestuff type ink.
In general, ink having the low permeation speed such as the pigment type ink is used as the overlay type ink. When the printing is carried out by the bi-directional multi-scan system while using the overlay type ink, the following problem may occur.
When the overlay type ink is used, lately printed ink is overlaid with earlier printed ink, and therefore, the image surface is dominated by the tint or glossiness of the lately printed ink. For this reason, in this text, the color of ink located at the uppermost surface is referred to as a dominant color.
Under such a condition, it is supposed that a uniform green image is printed with cyan ink having a relatively high degree of glossiness and yellow ink having a relatively low degree of glossiness by using a serial type ink jet printing apparatus. In this case, if the ink is applied in the order of cyan and yellow in the going direction, the ink is applied in the reverse order, that is, yellow and cyan, in the returning direction. Accordingly, an area printed in the going direction is dominated by yellow, and another area printed in the returning direction is dominated by cyan
When the multi-scan system is adopted, the dominant color is decided by the direction of the final printing scan. Therefore, the area in which the final printing scan is carried out in the going direction, yellow becomes the dominant color, while cyan becomes the dominant color in the area adjacent thereto As a result, the area in which yellow is the dominant color and the area in which cyan is the dominant color alternately appear on the image, whereby, in the visual sense, there is a problem in that areas having a high degree of glossiness and a low degree of glossiness appear as streaks to be felt as the glossy-banding. This phenomenon is a serious drawback of an image expected to have a high image quality.
The above-mentioned phenomenon in which areas having different dominant colors are alternately arranged simultaneously generates the color-banding. Accordingly, it is possible to use the technique already described in the prior art, such as Japanese Patent Nos. 3,200,143 or 3,236,034. According to the above patent documents, it is possible to approximately equalize a ratio of the dominant color in the respective image area as a pixel unit. That is, even if either yellow or cyan is a dominant color, the color-banding is reduced when the ratio of the dominant color is maintained approximately constant between the adjacent image areas.